Currently, prices are displayed without VAT and there is no indication that they are excluding VAT. As a customer, realizing that I am billed more than the advertised price makes me feel ripped off.

While not including VAT might be acceptable in some countries (or when targeting business customers and not private ones), it is definitely not acceptable in most of the world.

While displaying prices including VAT in the control panel if the account does not have a Tax ID entered is the preferred option, an alternative albeit less optimal solution would be to add an indication that advertised prices are inaccurate everywhere you display a price.